Heated oxygen sensor ground

[mrriggs]

I'm building an ECU from scratch to plug into a factory wiring harness on my LTD LX. In the EVTM it shows the ground for the oxygen sensor heater going into the ECU. Does anybody know off hand what the ECU does with this ground? I assume it either opens it when the sensor is warm or there is a current shunt to indicate a faulty heater.

What's interesting is that, according to the "EEC Bible" other vehicles had the heater permanently grounded outside of the ECU so it is always on. Is there any reason to turn off the heater once things are warmed up? I still have a couple unused pins on the microcontroller so it wouldn't be too hard to add in that function if it serves any advantage.

[Mikestang]

On my 86 with a A3M1 PCM, the O2 heater is powered all the time. On newer cars the Heater is ground controlled and PW drops off as the engine temp warms up. Some shut off completely, other stay around a 20% DC, depends on the calibration of the PCM. I assume this varies depending on O2 location in the exhaust.

[mrriggs]

Thanks, that's what I wanted to know.

I think I'll just ground it for now and not worry about switching it on and off.

[Haystack]

The factory ground is on the back of the head, just looped back from the plug.

Some 86's with sefi didn't have heated o2 sensors. My first 86 didn't, my second one did.

[mrriggs]

The EEC Bible shows a three-wire HEGO sensor on the '85 5.0 HO CFI. Pin-49 is the signal return and grounds to the back of the head.

The '85 LTD EVTM shows a four-wire HEGO sensor. The sensor ground is in the harness, not a separate wire to the head. The heater grounds in the ECU through pin-49.

I have the harness from an '85 LTD LX [not installed yet]. Broke out the DMM and started tracing wires. Big surprise, it doesn't match either diagram. It has a two-wire EGO sensor. Pin-49 is the signal return from the sensor.

So I guess I don't need to worry about an oxygen sensor heater after all.